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A computer network is a group of computers that are connected to each other for the
purpose of communication. Networks may be classified according to a
wide variety of characteristics. This article provides a general
overview of some types and categories and also presents the basic
components of a network.

Introduction

A computer network allows computers to communicate with many other
computers and to share resources and information. The Advanced
Research Projects Agency (ARPA) funded the
design of the "Advanced Research Projects Agency Network" (ARPANET) for the United States Department of
Defense. It was the first operational computer network in the
world. Development of the network began in 1969, based on designs
begun in the 1960s.

Network classification

The following list presents categories used for classifying
networks.

Connection method

Computer networks can also be classified according to the hardware
and software technology that is used to interconnect the individual
devices in the network, such as Optical
fiber, Ethernet, Wireless LAN, HomePNA,
Power line communication or
G.hn.Ethernet uses physical wiring to connect
devices. Frequently deployed devices include hubs, switches,
bridges and/or routers.

Wireless LAN technology is designed to connect devices without
wiring. These devices use radio waves or
infrared signals as a transmission
medium.

Twisted-Pair Wire - This is the most widely used medium
for telecommunication. Twisted-pair wires are ordinary telephone
wires which consist of two insulated copper wires twisted into
pairs and are used for both voice and data transmission. The use of
two wires twisted together helps to reduce crosstalk and electromagnetic induction. The
transmission speed range from 2 million bits per second to 100
million bits per second.

Coaxial Cable – These cables are widely used for cable
television systems, office buildings, and other worksites for local
area networks. The cables consist of copper or aluminum wire
wrapped with insulating layer typically of a flexible material with
a high dielectric constant, all of which are surrounded by a
conductive layer. The layers of insulation help minimize
interference and distortion. Transmission speed range from 200
million to more than 500 million bits per second.

Fiber Optics – These cables consist of one or more thin
filaments of glass fiber wrapped in a protective layer. It
transmits light which can travel over long distance and higher
bandwidths. Fiber-optic cables are not affected by electromagnetic
radiation. Transmission speed could go up to as high as trillions
of bits per second. The speed of fiber optics is hundreds of times
faster than coaxial cables and thousands of times faster than
twisted-pair wire.

Wireless Technologies

Terrestrial Microwave – Terrestrial microwaves use
Earth-based transmitter and receiver. The equipment look similar to
satellite dishes. Terrestrial microwaves use low-gigahertz range,
which limits all communications to line-of-sight. Path between
relay stations spaced approx. 30 miles apart. Microwave antennas
are usually placed on top of buildings, towers, hills, and mountain
peaks.

Communications Satellites – The satellites use microwave
radio as their telecommunications medium which are not deflected by
the Earth's atmosphere. The satellites are stationed in space,
typically 22,000 miles above the equator. These Earth-orbiting
systems are capable of receiving and relaying voice, data, and TV
signals.

Cellular and PCS Systems – Use several radio
communications technologies. The systems are divided to different
geographic area. Each area has low-power transmitter or radio relay
antenna device to relay calls from one area to the next area.

Wireless LANs – Wireless local area network use a
high-frequency radio technology similar to digital cellular and a
low-frequency radio technology. Wireless LANS use spread spectrum
technology to enable communication between multiple devices in a
limited area. Example of open-standard wireless radio-wave
technology is IEEE 802.11b.

Bluetooth – A short range wireless technology. Operate at
approx. 1Mbps with range from 10 to 100 meters. Bluetooth is an
open wireless protocol for data exchange over short
distances.

The Wireless Web – The wireless web refers to the use of
the World Wide Web through equipments like cellular phones,
pagers,PDAs, and other portable communications devices. The
wireless web service offers anytime/anywhere connection.

Scale

Networks are often classified as Local Area Network , Wide Area Network , Metropolitan Area Network , Personal Area Network , Virtual Private Network , Campus Area Network , Storage Area Network , etc. depending on their scale, scope
and purpose. Usage, trust levels and access rights often differ
between these types of network - for example, LANs tend to be
designed for internal use by an organization's internal systems and
employees in individual physical locations (such as a building),
while WANs may connect physically separate parts of an organization
to each other and may include connections to third parties.

Network topology

Computer networks may be classified according to the network topology upon which the network is
based, such as bus network, star network, ring
network, mesh network, star-bus network, tree or hierarchical topology
network. Network topology signifies the way in which devices in
the network see their logical relations to one another. The use of
the term "logical" here is significant. That is, network topology
is independent of the "physical" layout of the network. Even if
networked computers are physically placed in a linear arrangement,
if they are connected via a hub, the network has a Star topology,
rather than a bus topology. In this regard the visual and
operational characteristics of a network are distinct; the logical
network topology is not necessarily the same as the physical
layout.Networks may be classified based on the method of data used
to convey the data, these include digital and analog
networks.

Types of networks

Below is a list of the most common types of computer networks in
order of scale.

Personal area network

A personal area network (PAN)
is a computer network used for communication among computer devices
close to one person. Some examples of devices that are used in a
PAN are personal computers, printers, fax machines, telephones,
PDAs, scanners, and even video game consoles. Such a PAN may
include wired and wireless connections between devices. The reach
of a PAN is typically at least about 20-30 feet (approximately 6-9
meters), but this is expected to increase with technology
improvements.

Local area network

A local Area Network (LAN) is a
computer network covering a small physical area, like a home,
office, or small group of buildings, such as a school, or an
airport. Current wired LANs are most likely to be based on Ethernet technology, although new standards like
ITU-TG.hn also provide a
way to create a wired LAN using existing home wires (coaxial
cables, phone lines and power lines).

For example, a library may have a wired or wireless LAN for users
to interconnect local devices (e.g., printers and servers) and to
connect to the internet. On a wired LAN, PCs in the library are
typically connected by category 5
cable, running the IEEE 802.3 protocol through a system of
interconnected devices and eventually connect to the Internet. The
cables to the servers are typically on Cat 5e enhanced cable, which
will support IEEE 802.3 at 1 Gbit/s. A wireless LAN may exist using
a different IEEE protocol, 802.11b, 802.11g or possibly 802.11n.The
staff computers (bright green in the figure) can get to the color
printer, checkout records, and the academic network and
the Internet. All user computers can get to the Internet and the
card catalog. Each workgroup can get to its local printer. Note
that the printers are not accessible from outside their
workgroup.

All interconnected devices must understand the network layer (layer
3), because they are handling multiple subnets (the different
colors). Those inside the library, which have only 10/100 Mbit/s
Ethernet connections to the user device and a Gigabit Ethernet
connection to the central router, could be called "layer 3
switches" because they only have Ethernet interfaces and must
understand IP. It would be more
correct to call them access routers, where the router at the top is
a distribution router that connects to the Internet and academic
networks' customer access routers.

The defining characteristics of LANs, in contrast to WANs (Wide
Area Networks), include their higher data transfer rates, smaller
geographic range, and lack of a need for leased telecommunication
lines. Current Ethernet or other IEEE
802.3 LAN technologies operate at speeds up to 10 Gbit/s. This
is the data transfer rate. IEEE has projects
investigating the standardization of 40 and 100 Gbit/s.

Campus area network

A campus area network (CAN) is a
computer network made up of an interconnection of local area
networks (LANs) within a limited geographical area. It can be
considered one form of a metropolitan area network, specific to an
academic setting.

In the case of a university campus-based campus area network, the
network is likely to link a variety of campus buildings including;
academic departments, the university library and student residence
halls. A campus area network is larger than a local area network
but smaller than a wide area network (WAN) (in some cases).

The main aim of a campus area network is to facilitate students
accessing internet and university resources.This is a network that
connects two or more LANs but that is limited to a specific and
contiguous geographical area such as a college campus, industrial
complex, office building, or a military base. A CAN may be
considered a type of MAN (metropolitan area network), but is
generally limited to a smaller area than a typical MAN. This term
is most often used to discuss the implementation of networks for a
contiguous area. This should not be confused with a Controller Area Network.A LAN
connects network devices over a relatively short distance. A
networked office building, school, or home usually contains a
single LAN, though sometimes one building will contain a few small
LANs (perhaps one per room), and occasionally a LAN will span a
group of nearby buildings.

Metropolitan area network

A metropolitan area
network (MAN) is a network that connects two or more local area
networks or campus area networks together but does not extend
beyond the boundaries of the immediate town/city. Routers, switches
and hubs are connected to create a metropolitan area network.

Wide area network

A wide area network (WAN) is a
computer network that covers a broad area (i.e. any network whose
communications links cross metropolitan, regional, or national
boundaries [1]). Less formally, a WAN is a network that uses
routers and public communications links. Contrast with personal
area networks (PANs), local area networks (LANs), campus area
networks (CANs), or metropolitan area networks (MANs), which are
usually limited to a room, building, campus or specific
metropolitan area (e.g., a city) respectively. The largest and most
well-known example of a WAN is the Internet.A WAN is a data
communications network that covers a relatively broad geographic
area (i.e. one city to another and one country to another country)
and that often uses transmission facilities provided by common
carriers, such as telephone companies. WAN technologies generally
function at the lower three layers of the OSI
reference model: the physical
layer, the data link layer, and
the network layer.

Global area network

A global area networks (GAN) (see also IEEE
802.20) specification is in development by several groups, and
there is no common definition. In general, however, a GAN is a
model for supporting mobile communications across an arbitrary
number of wireless LANs, satellite coverage areas, etc. The key
challenge in mobile communications is "handing off" the user
communications from one local coverage area to the next. In IEEE
Project 802, this involves a succession of terrestrial WIRELESS local area networks .

Virtual private network

A virtual private network
(VPN) is a computer network in which some of the links between
nodes are carried by open connections or virtual circuits in some
larger network (e.g., the Internet) instead of by physical wires.
The data link layer protocols of the virtual network are said to be
tunneled through the larger network when this is the case. One
common application is secure communications through the public
Internet, but a VPN need not have explicit security features, such
as authentication or content encryption. VPNs, for example, can be
used to separate the traffic of different user communities over an
underlying network with strong security features.

A VPN may have best-effort performance, or may have a defined
service level agreement (SLA) between the VPN customer and the VPN
service provider. Generally, a VPN has a topology more complex than
point-to-point.

A VPN allows computer users to appear to be editing from an IP
address location other than the one which connects the actual
computer to the Internet.

Internetwork

An Internetwork is the connection of
two or more distinct computer networks or network segments via a
common routing technology. The result is called an internetwork
(often shortened to internet).Two or more networks or network
segments connect using devices that operate at layer 3 (the
'network' layer) of the OSI Basic Reference Model, such as a
router. Any interconnection among or between public, private,
commercial, industrial, or governmental networks may also be
defined as an internetwork.

In modern practice, interconnected networks use the Internet
Protocol. There are at least three variants of internetworks,
depending on who administers and who participates in them:

Intranet

Extranet

Internet

Intranets and extranets may or may not have connections to the
Internet. If connected to the Internet, the intranet or extranet is
normally protected from being accessed from the Internet without
proper authorization. The Internet is not considered to be a part
of the intranet or extranet, although it may serve as a portal for
access to portions of an extranet.

Intranet

An intranet is a set of networks, using the
Internet Protocol and IP-based
tools such as web browsers and file transfer applications, that is
under the control of a single administrative entity. That
administrative entity closes the intranet to all but specific,
authorized users. Most commonly, an intranet is the internal
network of an organization. A large intranet will typically have at
least one web server to provide users with organizational
information.

Extranet

An extranet is a network or internetwork
that is limited in scope to a single organization or entity and
also has limited connections to the networks of one or more other
usually, but not necessarily, trusted organizations or entities
(e.g., a company's customers may be given access to some part of
its intranet creating in this way an extranet, while at the same
time the customers may not be considered 'trusted' from a security
standpoint). Technically, an extranet may also be categorized as a
CAN, MAN, WAN, or other type of network, although, by definition,
an extranet cannot consist of a single LAN; it must have at least
one connection with an external network.

Basic hardware components

All networks are made up of basic hardware building blocks to
interconnect network nodes, such
as Network Interface Cards (NICs), Bridges, Hubs, Switches, and
Routers. In addition, some method of connecting these building
blocks is required, usually in the form of galvanic cable (most
commonly Category 5 cable). Less
common are microwave links (as in IEEE
802.12) or optical cable ("optical
fiber"). An Ethernet card may also be required.

Network interface cards

A network card, network adapter, or NIC
(network interface card) is a piece of computer hardware designed to allow
computers to communicate over a computer network. It provides
physical access to a networking medium and often provides a
low-level addressing system through the use of MAC addresses.

Repeaters

A repeater is an electronic device that receives a signal and retransmits it at a higher power level, or to the
other side of an obstruction, so that the signal can cover longer
distances without degradation. In most twisted pair Ethernet
configurations, repeaters are required for cable which runs longer
than 100 meters.

Hubs

A network hub contains multiple ports.
When a packet arrives at one port, it is copied unmodified to all
ports of the hub for transmission. The destination address in the
frame is not changed to a broadcast address.

Bridges

A network bridge connects multiple
network segments at the data link layer (layer 2) of the OSI model. Bridges do not promiscuously copy
traffic to all ports, as hubs do, but learn which MAC addresses are reachable through specific
ports. Once the bridge associates a port and an address, it will
send traffic for that address only to that port. Bridges do send
broadcasts to all ports except the one on which the broadcast was
received.

Bridges learn the association of ports and addresses by examining
the source address of frames that it sees on various ports. Once a
frame arrives through a port, its source address is stored and the
bridge assumes that MAC address is associated with that port. The
first time that a previously unknown destination address is seen,
the bridge will forward the frame to all ports other than the one
on which the frame arrived.

Bridges come in three basic types:

Local bridges: Directly connect local area networks (LANs)

Remote bridges: Can be used to create a wide area network (WAN)
link between LANs. Remote bridges, where the connecting link is
slower than the end networks, largely have been replaced with
routers.

Wireless bridges: Can be used to join LANs or connect remote
stations to LANs

Switches

A network switch is a device that
forwards and filters OSI layer 2datagrams (chunk of data communication)
between ports (connected cables) based on the MAC addresses in the
packets. This is distinct from a hub in that it only forwards the
packets to the ports involved in the communications rather than all
ports connected. Strictly speaking, a switch is not capable of
routing traffic based on IP address (OSI Layer 3) which is
necessary for communicating between network segments or within a
large or complex LAN. Some switches are capable of routing based on
IP addresses but are still called switches as a marketing term. A
switch normally has numerous ports, with the intention being that
most or all of the network is connected directly to the switch, or
another switch that is in turn connected to a switch.

Switch is a marketing term that encompasses routers and bridges, as
well as devices that may distribute traffic on load or by
application content (e.g., a Web URL identifier). Switches may
operate at one or more OSI model layers,
including physical, data link, network, or transport . A device that operates
simultaneously at more than one of these layers is called a
multilayer switch.

Overemphasizing the ill-defined term "switch" often leads to
confusion when first trying to understand networking. Many
experienced network designers and operators recommend starting with
the logic of devices dealing with only one protocol level, not all
of which are covered by OSI. Multilayer device selection is an
advanced topic that may lead to selecting particular
implementations, but multilayer switching is simply not a
real-world design concept.

Routers

A router is a networking device that forwards
packets between
networks using information in protocol headers and forwarding
tables to determine the best next router for each packet. Routers
work at the Network Layer (layer 3) of
the OSI model and the Internet Layer of TCP/IP.